The gastrointestinal tract represents the largest surface area of direct contact between the external environment and the mucosal immune system. T cells accumulate in the gut-associated lymphoid tissue (GALT), including the mesenteric lymph nodes, Peyer's patches, lymphoid follicles, and are scattered throughout the lamina propria and intestinal epithelium. In germ-free mice GALT cellularity is reduced, indicative of the important influence of intestinal symbionts on T cell accumulation in mucosal lymphoid organs. Intestinal bacteria generate immunomodulatory metabolites that influence lymphocyte trophism and effector functions. The intestinal microbiota is also a major contributor of antigens and the diversity of these antigens varies significantly across the intestine. Mucosal T cells can recognize luminal antigens, but how this recognition affects clonal distribution of CD4+ T cells and their effector function is currently unknown. In the intestine, in addition to the resident commensal microflora, the immune system is exposed to continual challenge by dietary antigens, and occasionally pathogenic microbes. A balance between tolerance towards commensals, food antigens and immune reactivity towards pathogens is critical to the maintenance of intestinal homeostasis. One important mechanism responsible for the maintenance of this balance is the suppressive CD4+Foxp3+ regulatory T cells (Tregs) towards effector T cells. Here, we propose to study an impact of initial bacterial colonization with symbiotic bacteria on the diversity of TCRs on intestinal Tregs (Specific Aim 1). In our Specific Aim 2, we will compare Tregs participation in immunoresponse to infectious bacteria in commensal-competent and incompetent mice. We will also determine if different subsets of Tregs derived from nave CD4+ cells (adaptive Tregs and Tr1 producing IL-10) have complementary or redundant roles in intestinal homeostasis. Finally in our last aim we will study how frequently Tregs change their phenotype to effector Th17 and follicular lineages in pro- inflammatory milieu.